Final Diagnosis -- Primary amebic meningoencephalitis



The decedent was a 19 year old previously healthy male who developed meningitis type symptoms several days after the jet ski accident in which he fell into stagnant water of a man made lake (fresh water) off the intercostal water way (salt water) engulfing water and sediment an sustaining minor superficial injuries. CSF cultures were negative for bacterial organisms, fungi, etc. Neutrophils and protein were markedly elevated with low glucose. Aggressive treatment for a presumed bacterial illness were without avail and he expired 6 days after the onset of the febrile illness. In light of the clinical presentation with the initial incident in stagnant water (later confirmed to be a man-made fresh water lake), suspicion for an amebic etiology of the decease process was raised. Immunofluorescent tests were carried out by the CDC in Atlanta, GA on tissue slides obtained post-mortem using antisera against Nagleria Fowleri, Acanthamoeba Castellanii and Balamuthia Mandrillari (3 amebas known to cause amebic meningoencephalitis in humans). The amebas on the sections reacted only with the anti-N.Fowleri serum.1. The histiocyte like cells were identified as amebas. They resemble macrophages. Their main distinguishing features are spherical vesicular nuclei and uniformly prominent nucleoli1 (Figure 4, arrows). The route of infection is usually through the nose to the meninges with early exudate appearing around olfactory bulbs (which was not the case here), with spread throughout the subarachnoid space and intracerebral extension2.

Infection by the free living ameba Nagleria Fowleri occurs after exposure to polluted water in man-made fresh water lakes, ponds, swimming pools, particularly during the warm weather months when the thermophilic ameba grows well3. The pathologic substrate of the infection is an acute hemorrhagic necrotizing meningo-encephalitis mainly at the base of the brain, brainstem and cerebellum occurring in young, healthy individuals. Clusters of amebic trophozoites are seen around blood vessels. They measure 8 to 12 micrometers in diameter3 179 cases have been reported world wide, 81 in the USA by October 19963.

Other free-living opportunistic protozoa form the main differential diagnosis: Acanthamoeba Castellani and Balamuthia Mandrellaris develop cysts in the brain and cause a chronic granulomatous amoebic encephalitis (GAE) in humans, mostly in debilitated, malnourished immunosuppressed individuals. These amoebas cause a subacute or chronic granulomatous encephalitis. Exposure occurs through the respiratory tract or skin lesion with hematogenous spread through the CNS5. Invasion is facilitated by a cysteine protease capable of destroying host tissue6


  1. Visvesvara GS (personal correspondence). Immunofluorescence test on decedent's tissue slides. Center for Disease Control, Atlanta, GA 2/24/2000
  2. Earle K M. Case 23. 40th Annual Anatomic Pathology Seminar ASCP, October 1974. Published by ASCP, Chicago, 1976.
  3. Martinez AJ, Visvesvara GS. Free-living, Amphozoic and opportunistic amebas. Brain Pathology 1997; 7:583-598.
  4. Scaravilli F, Cook GC. Parasitic and fungal infections in Greenfield's Neuropathology, 6th ed, 1997. Vol II. Chapter2, Pp 65-111.
  5. Martinez AJ, Duma RJ, Nelson EC, et al. Experimental Naegleria encephalitis in mice. Penetration of the olfactory mucosal epithelium by Naegleria and pathologic changes produced. Lab. Invest. 1973, 29:121-133.
  6. Aldope K, Huizinga H, Bouvier J. et al. Naegleria fowleri: Characterization of a secreted histolytic cysteine protease. Exp. Parasitol 1994; 78:230-41

Contributed by Eva Gyori, MD
Acknowledgements to Dr. Michael Hunter and Mr. Lenny Wolf for their contributions

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